#define _CRT_SECURE_NO_WARNINGS 

class Solution {
public:
    string strA[10] = { "","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz" };
    void Combina(string digits, int level, string Combinastr, vector<string>& v)
    {
        if (level == digits.size())
        {
            v.push_back(Combinastr);
            return;
        }
        int num = digits[level] - '0';
        string str = strA[num];
        for (size_t i = 0; i < str.size(); i++)
        {
            Combina(digits, level + 1, Combinastr + str[i], v);
        }

    }
    vector<string> letterCombinations(string digits) {
        vector<string> v;
        if (digits.empty())
        {
            return v;
        }
        Combina(digits, 0, "", v);
        return v;
    }
};